/Engine/Source/Editor/MainFrame/Public/Interfaces/IEditorMainFrameProvider.h
Spawns a widget for slotting into the main editor window (in-place of the standard editor).
A replacement widget for the editor's main window.
[TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > CreateMainFrameContentWidget() const